A new scientific study concludes that changes in the Sun's output cannot be causing modern-day climate change.

It shows that for the last 20 years, the Sun's output has declined, yet temperatures on Earth have risen.

It also shows that modern temperatures are not determined by the Sun's effect on cosmic rays, as has been claimed.

Writing in the Royal Society's journal Proceedings A, the researchers say cosmic rays may have affected climate in the past, but not the present.

"This should settle the debate," said Mike Lockwood, from the UK's Rutherford-Appleton Laboratory, who carried out the new analysis together with Claus Froehlich from the World Radiation Center in Switzerland.

This paper re-enforces the fact that the warming in the last 20 to 40 years can't have been caused by solar activity

Dr Piers Forster

Dr Lockwood initiated the study partially in response to the TV documentary The Great Global Warming Swindle, broadcast on Britain's Channel Four earlier this year, which featured the cosmic ray hypothesis.

"All the graphs they showed stopped in about 1980, and I knew why, because things diverged after that," he told the BBC News website.

"You can't just ignore bits of data that you don't like," he said.

Warming trend

The scientists' main approach on this new analysis was simple: to look at solar output and cosmic ray intensity over the last 30-40 years, and compare those trends with the graph for global average surface temperature, which has risen by about 0.4C over the period.

Temperatures have continued rising irrespective of cosmic ray flux

The Sun varies on a cycle of about 11 years between periods of high and low activity.

But that cycle comes on top of longer-term trends; and most of the 20th Century saw a slight but steady increase in solar output.

However, in about 1985, that trend appears to have reversed, with solar output declining.

Yet this period has seen temperatures rise as fast as - if not faster than - any time during the previous 100 years.

"This paper reinforces the fact that the warming in the last 20 to 40 years can't have been caused by solar activity," said Dr Piers Forster from Leeds University, a leading contributor to this year's Intergovernmental Panel on Climate Change (IPCC) assessment of climate science.

Cosmic relief

The IPCC's February summary report concluded that greenhouse gases were about 13 times more responsible than solar changes for rising global temperatures.

But the organisation was criticised in some quarters for not taking into account the cosmic ray hypothesis, developed by, among others, Henrik Svensmark and Eigil Friis-Christensen of the Danish National Space Center.